Conventional valve operating systems consist of combinations of camshafts, push-rods or push-tubes, rocker-arms, and valve-lifters. Camshaft rotation is mechanically slaved to the crankshaft rotation of the engine. Consequently, valve openings, and closings, are fixed to the rotation of a crankshaft. Full authority is defined as having the ability to control the openings, and closings, of an internal combustion engine's intake and exhaust valves independent of the engine's crankshaft rotational position.
The control of exhaust and intake valves of four-cycle internal combustion engines, conventionally, is achieved by mechanical or hydromechanical systems operated synchronously with the engine's crankshaft rotation. The timing of valve openings is fixed in relationship to the crankshaft's position by direct mechanical linking of the valve actuating system with the crankshaft. In any cylinder, of a multi-cylinder internal combustion engine, intake and exhaust valve openings and closings, in conjunction with the fuel mixture and either ignition or fuel injection, are predetermined to provide optimum positive power over a range of engine speeds.
Oil aeration occurs when oil and air are mixed together. Aeration encompasses four distinct forms: air dissolved in oil, entrained air, bulk air, and foam. Bulk air refers to large masses of trapped air, such as air pockets. In a brake housing this situation is created when oil leaks out around pistons and set screws after the engine has sat for a duration of time. Air dissolved in oil assumes a position in the chemical matrix of oil. Entrained air describes pockets of air that are uniformly suspended throughout oil yet are not assimilated into oil like dissolved air. The delineating factor between entrained and bulk air is the size of the voids each creates. Typically, everything 1 mm in diameter and less is defined as entrained air. Hence, bulk air assumes everything larger than 1 mm in diameter. Finally, foam is the occurrence of a separate layer of air bubbles on the surface of a fluid.
Fully hydraulic valve actuation systems are subject to aeration of the oil. Prolonged sedentary states of the engine also promote the accumulation of bulk air in the hydraulic housing due to leak down. Bulk air in the system, especially in the start up mode, may adversely impact the valve motion. In particular, depending on the amount of air entrapped in the system at start-up, the entrapped air may delay or possibly preclude valve opening in response to the hydraulic fluid being pumped through the system. Without the removal of the existing bulk air, the system may become paralyzed or function improperly, which may cause valve opening at non-prescribed timings.